The need for a hydrogen sensor with high sensitivity, fast regeneration, and an even faster response time is gaining momentum as efforts to develop a hydrogen economy continue to grow. Numerous companies and organizations such as NASA and DOE, that use large quantities of hydrogen and oversee the development of the technology, have outlined a detailed performance criterion for an acceptable hydrogen sensor. Numerous approaches are being investigated to develop these hydrogen sensors, including sol-gel-based sensors, semiconductor sensors, oxide-based sensors, thin-film-based sensors and acoustic wave sensors. These techniques generally either require a lot of power, show a slow response time or lack the required sensitivity. Recently nanowire-based sensors for detecting hydrogen have been reported. These sensors nanowire-based sensors have been shown to respond in real time. However they lack the sensitivity needed and do not respond to low concentrations of hydrogen. Additionally, the techniques used to fabricate these nanowire sensors require complex procedures, such as the transfer of nanostructures and their organized assembly. These complex fabrication methodologies add to the cost of manufacture making nanowire sensors unsuitable for commercial production.
Porous silicon substrates have been employed in the past to build functional hydrogen sensors. Hydrogen sensors are known in the art wherein the absorption of hydrogen results in the expansion of a palladium (Pd) lattice and a change in the refractive index results such that hydrogen can be detected using optical interferometric techniques.
Palladium is an ideal material for hydrogen sensing because it selectively absorbs hydrogen gas and forms a chemical species known as a palladium hydride. Thick-film hydrogen sensors are known in the art that rely on the fact that palladium metal hydride's electrical resistance is greater than the metal's resistance. In such systems, the absorption of hydrogen is accompanied by a measurable increase in electrical resistance. The resistance increase is caused by the increased resistivity of palladium hydride relative to pure palladium.
By contrast, palladium thin-film sensors as are known in the art are based on an opposing property that depends on the nanoscale structures within the thin film. In the thin film, nanosized palladium particles, or nanoclusters, swell when the hydride is formed, and in the process of expanding, some of the nanoclusters form new electrical connections with neighboring nanoclusters. The increased number of conducting pathways results in an overall net decrease in resistance.
In view of the palladium based sensors known in the art, it is desirable to provide a device that is sensitive to hydrogen, and in particular a hydrogen gas sensor that is easy to fabricate and that exhibits fast regeneration, high sensitivity and a fast response time.